The present invention relates to DC-DC voltage converters, and more particularly to DC-DC voltage converters having an output voltage greater than the input voltage.
DC-DC voltage converters are known in the art that generate an output voltage that is greater than the input voltage. Such converters utilize input and output capacitors, an inductor, one or more diodes, a switching transistor, and a pulse width modulator, generally implemented by an integrated circuit. Such converters require some type of voltage feedback to regulate the output voltage. The voltage feedback loop requires additional resistors and capacitors and must be designed so that the output voltage remains stable under varying load conditions. These prior art voltage converters usually require other external parts to provide soft start and current limit.
Examples of prior art DC-DC converters include a flyback circuit (FIG. 1), a flyback circuit with transformer isolation (FIG. 2) and a boost circuit (FIG. 3). Flyback circuits inherently have poor voltage regulation. Absent feedback, the output voltage of a flyback circuit tends to increase with decreasing load, and if no load is present, the output voltage may rise to unacceptable levels. The flyback circuit of FIG. 1 incorporates a feedback circuit to compensate for variations in output voltage due to variations in input voltage and varying load conditions, nevertheless, its output voltage is negative with respect to its input voltage, which may require the converter to be isolated from system ground.
The flyback circuit of FIG. 2 incorporates an isolation transformer which overcomes the voltage polarity problem of the circuit of FIG. 1, however, it still exhibits substantially the same poor voltage regulation characteristics.
The boost circuit of FIG. 3 utilizes a series inductance to generate a voltage additive to the input voltage. This circuit also exhibits poor voltage regulation and has the additional drawback that the series inductor must be sized to carry full load current.
The present invention is a DC-DC converter that does not utilize a feedback loop, yet is stable, operates from no load to full load with good voltage regulation, and provides a xe2x80x9csoftxe2x80x9d start by initially applying to the load a low voltage substantially equal to its input voltage and then providing a controlled ramp-up to full output voltage. Although there are several alternative embodiments of the present invention, the preferred embodiment uses a field programmable gate array (FPGA) to provide two pulse width modulation (PWM) control signals that alternately drive a pair of semiconductor switches, preferably MOSFETs, between conducting and nonconducting states. Each of the semiconductor switches is connected between ground and one end of the winding of an autotransformer. Input voltage is provided between a center tap of the autotransformer winding and ground. Two rectifying diodes are connected in a common cathode configuration with an anode of each diode being connected to an end of the autotransformer winding. The positive terminal of the output is provided at the common cathode connection of the diodes and the negative terminal at ground.